AL2O3-SiO2-based oxide phosphor

ABSTRACT

An oxide phosphor that is highly durable and produces visible light when excited by exposure to near-ultraviolet excitation light, comprising an oxide having the composition represented by the formula (Al 2 O 3 ) x .(SiO 2 ) 1-x , where 0&lt;x&lt;1, and an activating element M.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present embodiment relates to a phosphor that produces fluorescencewhen excited by exposure to excitation light and, in particular, anAl₂O₃—SiO₂-based oxide phosphor.

2. Description of the Related Art

Phosphors that produce visible light when excited by exposure tonear-ultraviolet light are used in, for example, luminescent apparatusesconstituted of such a phosphor and a source of near-ultraviolet light,such as a semiconductor LED device, in combination therewith (e.g., seeJapanese Unexamined Patent Application Publication No. 2007-103512).

However, such phosphors have some problems. For example, they are basedon expensive complex oxides of decreasing availability or otherwiseunsatisfactory in terms of durability.

This unsatisfactory situation causes demand for a novel phosphor that ishighly durable and produces visible light when excited by exposure tonear-ultraviolet excitation light.

SUMMARY OF THE INVENTION

The inventors of the present invention conducted extensive research andfound that an Al₂O₃—SiO₂-based oxide generates fluorescence whenactivated by an activator, thereby completing the present embodiment.

The phosphor according to the present embodiment, an Al₂O₃—SiO₂-basedoxide that is easily available and has a high melting point, excellentinsulation properties, a high mechanical strength, and a low thermalexpansion coefficient is used as a host material, thereby enablingproducing highly durable phosphors at low cost.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a line plot that represents the relationship between thecontent ratio of Eu (mol %) and the fluorescent intensity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following are detailed descriptions of the present embodiment.

The phosphor according to the present embodiment uses anAl₂O₃—SiO₂-based oxide as a host material. In particular, anAl₂O₃—SiO₂-based oxide having a composition of(Al₂O₃)_(x).(SiO₂)_(1-x)(0.5≦x≦0.7) provides a high fluorescentintensity.

In the embodiment, a high fluorescent intensity can be achieved when theAl₂O₃—SiO₂-based oxide comprises mullite which is the only stablecompound. There are no limitations on Mullite composition. Thecomposition may be, for example, (Al₂O₃)_(x).(SiO₂)_(1-x)(0.5≦x≦0.7) or(Al₂O₃)_(x).(SiO₂)_(1-x)(0.6≦x≦0.67).

Also, there are no limitations on mullite content in theAl₂O₃—SiO₂-based oxide. The content may be, for example, 50 mass % orhigher, 75 mass % or higher, 90 mass % or higher, or 100 mass %.

For example, an embodiment of the Al₂O₃—SiO₂-based oxide includesmullite having a composition of 3Al₂O₃.2SiO₂, 3Al₂O₃.1.75SiO₂, or3Al₂O₃.1.5SiO₂ or a mixture thereof.

The phosphor according to the present embodiment is activated by anactivating element, M. Examples of the activating element M include Agand lanthanoids, such as Ce, Sm, Eu, Yb, and Dy. In particular, Ce andEu would be advantageous in ensuring luminance efficiency.

The activating element M may exist as divalent or trivalent ions in thephosphor according to the present embodiment. In particular, theluminous efficiency of the phosphor tends to become high when thelanthanoids exist as divalent ions.

The activating element M may take any form in the phosphor according tothe present embodiment as long as it is contained in theAl₂O₃—SiO₂-based oxide. It may be incorporated into the Al₂O₃—SiO₂-basedoxide to form a solid solution or it may exist in the grain boundariesor between the crystalline layers of the Al₂O₃—SiO₂-based oxide.

There are no limitations on the content of the activating element M, andthe content may be, for example, in the range of 0.001 to 2.0 mol %,0.001 to 1.0 mol %, 0.01 to 0.5 mol %, or 0.01 to 0.2 mol %, relative tothe total amount(mol) of Al and Si.

A method for manufacturing the phosphor according to the presentembodiment is described below.

There are no limitations on the production method for the phosphoraccording to the present embodiment, and it may be produced by, forexample, mixing an activating element M source and an Al₂O₃—SiO₂-basedoxide and/or a starting materials thereof followed by sintering themixture.

There is no limitation on the sintering temperature. When anAl₂O₃—SiO₂-based oxide and an activating element M are mixed andsintered, the sintering temperature may be, for example, a temperatureequal to or lower than the melting point of the Al₂O₃—SiO₂-based oxide.More specifically, the sintering temperature may be in the range of from1000 to 1800° C., from 1100 to 1700° C., or from 1200 to 1600° C.

Similarly, there is no limitation on the sintering time. The sinteringtime may be in the range of from a few hours to a dozen hours, or fromone to ten hours, or from three to seven hours.

There is no limitation on the sintering atmosphere, and the sinteringprocess may be conducted, for example, in vacuum, in a nitrogenatmosphere, or in an air atmosphere. In the present embodiment, in thecase where the activating element is a lanthanoid, the fluorescentintensity tends to become high when the sintering process is conductedin a reducing atmosphere, such as a hydrogen atmosphere. The reason forthis is unclear; however, it is presumed that when the sintering processis conducted in a reducing atmosphere,lanthanoid atoms would beincorporated in the Al₂O₃—SiO₂-based oxide as divalent ions.

There is no limitation on the preparation method of the Al₂O₃—SiO₂-basedoxide, and it may be prepared by any known method. For example, theAl₂O₃—SiO₂-based oxide may be prepared by a sol-gel method.

In addition, the Al₂O₃—SiO₂-based oxide may be prepared at the same timeas the phosphor by mixing starting materials of the oxide (e.g., Al andSi) with a lanthanoid atoms source and then sintering the mixture.

When mullite is used as the Al₂O₃—SiO₂-based oxide, the oxide may beprovided in the form of mullite crystals, or may be provided as amullite precursor, such as an amorphous phase and then be transformedinto mullite crystals by adjusting a sintering conditions or coolingconditions in the sintering step after being mixed with an activatingelement.

The activating element source to be used in the production of thephosphor according to the present embodiment is not limited, andexamples thereof include an element alone, oxide, hydroxide, chloride,fluoride, carbonate, sulfate, nitrate, and acetate of the activatingelement.

A luminescent apparatus such as a plasma screen and a whitelight-emitting diode can be provided by combining the oxide phosphoraccording to the present embodiment and a luminous device that generateslight exciting the phosphor.

The present embodiment is illustrated in detail by the followingexamples; however, these examples impose no limitation on the presentembodiment.

EXAMPLE 1

1. Synthesis of a Mullite Precursor(3Al₂O₃.2SiO₂[(Al₂O₃)_(0.6).(SiO₂)_(0.4)])

A mullite precursor was synthesized using a sol-gel method as follows.

A solution of 135 g of aluminum nitrate nonahydrate in 200 mL of ethanoland a solution of 27.6 mL of tetraethyl orthosilicate in 200 mL ofethanol were added in a round-bottom flask, and then 160 mL of anammonium hydroxide solution (25 mass %) was added thereto. This mixturewas heated at 60° C. and hydrolyzed for two hours while being stirred.Then, the obtained amorphous gel was heated at 110° C. for the removalof the solvent and then preliminary sintered at 300° C. The resultingproduct was transferred in an electric furnace and sintered under an airatmosphere at 1200° C. for two hours to give a mullite precursor.

2. Production of a Phosphor

In ethanol, 2.0 g of the obtained mullite precursor was wet-mixed with0.017 g of Eu₂O₃. The mixture was dried naturally, transferred in analumina crucible, and sintered under a reducing atmosphere with ahydrogen gas flow (95% of nitrogen and 5% of hydrogen), at 1500° C. forfive hours with the electric furnace used in Step 1.

3. Identification of the Sample

The obtained sample was subjected to a wet chemical analysis based onalkali fusion and, the constituent elements were proved to be Al, Si,Eu, and O.

Subsequent X-ray powder diffractometry using an X-ray diffractometer(Shimadzu Corporation, XD-D1 [Cu-Kα radiation]) revealed that the sampleconsisted of a single-phase mullite (3Al₂O₃.2SiO₂) and contained noother crystalline phases. Therefore, it is presumed that Eu wasincorporated into the mullite crystals to form a solid solution.

4. Evaluation of Fluorescence Properties

The obtained sample was evaluated for fluorescence properties using afluorescent spectrophotometer (Hitachi, Ltd., F-3000). The maximumexcitation wavelength and maximum emission wavelength are shown in thetable.

EXAMPLE 2

A phosphor was produced in the same way as Example 1 except that, inStep 2 , an air atmosphere was used instead of a reducing atmospherewith a hydrogen gas flow at the time of sintering the mixture of themullite precursor and Eu₂O₃.

The obtained sample consisted of the same elements as Example 1, and thecrystal structure thereof was a single phase-mullite. The maximumexcitation wavelength and maximum emission wavelength measured for thissample are also shown in the table.

EXAMPLE 3

A phosphor was produced in the same way as Example 1 except that 0.049 gof Sm₂O₃ was used instead of Eu₂O₃.

The crystal structure of the obtained sample was a single-phase mullite.The maximum excitation wavelength and maximum emission wavelengthmeasured for this sample are also shown in the table.

EXAMPLE 4

A phosphor was produced in the same way as Example 3 except that, inStep 2, an air atmosphere was used instead of a reducing atmosphere witha hydrogen gas flow at the time of sintering the mixture of the mulliteprecursor and Sm₂O₃.

The crystal structure of the obtained sample was a single-phase mullite.The maximum excitation wavelength and maximum emission wavelengthmeasured for this sample are also shown in the table.

EXAMPLE 5

A phosphor was produced in the same way as Example 1 except that 0.024 gof CeO₂ was used instead of Eu₂O₃.

The crystal structure of the obtained sample was a single-phase mullite.The maximum excitation wavelength and maximum emission wavelengthmeasured for this sample are also shown in the table.

EXAMPLE 6

A phosphor was produced in the same way as Example 1 except that 0.055 gof Yb₂O₃ was used instead of Eu₂O₃.

The crystal structure of the obtained sample was a single-phase mullite.The maximum excitation wavelength and maximum emission wavelengthmeasured for this sample are also shown in the table.

EXAMPLE 7

A phosphor was produced in the same way as Example 1 except that 0.052 gof Dy₂O₃ was used instead of the Eu₂O₃.

The crystal structure of the obtained sample was a single-phase mullite.

EXAMPLE 8

A phosphor was produced in the same way as Example 1 except that 2 g ofthe mullite precursor was mixed with 0.2 g of Ag₂SO₄ and then 0.0020 gof the obtained mixture was further mixed with 2 g of the mulliteprecursor and the resulting mixture was used as a starting material.

The crystal structure of the obtained sample was a single-phase mullite.

The table below lists the maximum excitation wavelengths, maximumemission wavelengths, and the visually observed fluorescence color ofthe individual samples.

TABLE 1 Maximum Maximum Fluo- Sintering excitation emission res-Activating atmos- wavelength wavelength cence element phere (nm) (nm)color Example 1 Eu Hydrogen 307 454 Blue Example 2 Air 307 405 PurpleExample 3 Sm Hydrogen 368 757 Red Example 4 Air 368 757 Red Example 5 CeHydrogen 307 411 Blue Example 6 Yb Hydrogen 314 454 Blue Example 7 DyHydrogen Not Not White determined determined Example 8 Ag Hydrogen NotNot Blue determined determined

When exposed to near-ultraviolet excitation light, the phosphors ofExamples 1 to 8 were all excited and emitted visible light. Inparticular, the examples containing Eu or Sm as the activating elementemitted intense light.

In the examples comprising Eu as the activating element the maximumemission wavelength of the phosphor sintered in a hydrogen gas flow wasdifferent from that of the phosphor sintered in an air atmosphere. Thisis probably because sintering in a hydrogen gas flow(a reducingatmosphere) reduced Eu³⁺ to Eu²⁺. On the other hand, in the examplescomprising Sm as the activating element, the difference in the sinteringatmosphere had no effect on the maximum emission wavelength of theresulting phosphors. This is probably because Sm³⁺ is very likely to bereduced and thus it is reduced to Sm²⁺ by being sintered even in an airatmosphere.

EXAMPLE 9

Two mullite precursors, 3Al₂O₃.1.75SiO₂ and 3Al₂O₃.1.5SiO₂, wereprepared in addition to 3Al₂O₃.2SiO₂ in the same way as Example 1 exceptthat the additive ratio of aluminum nitrate and tetraethyl orthosilicateused in the synthesis of the mullite precursor was modified.

Subsequently, phosphors were produced from each of the three mulliteprecursors in the same way as Example 1 except that the additive amountof Eu₂O₃ mixed with the mullite precursor was modified so that thecontent ratio of Eu in each of the resulting phosphors was varied. Thefluorescent intensity (relative intensity) of the obtained phosphorswere measured, and the results was shown in FIG. 1. It should be notedthat, in FIG. 1, the vertical axis represents the ratio of thefluorescent intensity of each of the phosphors to the maximum valueamong each of the measured intensities (relative intensity of eachphosphor), whereas the horizontal axis represents the content ratio ofEu (mol %)(percentage relative to the total amount (mol) of Al and Si).

As seen in FIG. 1, the relationship between the content ratio of Eu andthe fluorescent intensity was similar among the phosphors regardless ofthe difference in the composition of the mullite as the host material.Furthermore, every sample exhibited the maximum fluorescent intensitywhen the content ratio of Eu was approximately 0.06 mol %, and thefluorescent intensity no longer increased with an increase in thecontent ratio of Eu.

Reference Example

Phosphors were produced from three mullite precursors, 3Al₂O₃.2SiO₂,3Al₂O₃.1.75SiO₂, and 3Al₂O₃.1.5SiO₂, in the same way as Example 9 exceptthat the additive amount of EU₂O₃ was modified so that the contentratios of Eu in the resulting phosphors were varied. X-ray powderdiffractometry of the obtained phosphors revealed that the phosphorswith a content ratio of Eu to the mullite (percentage relative to thetotal amount (mol) of Al and Si) of 0.4 mol % or lower consisted of asingle-phase mullite, regardless of the composition of mullite. However,when the content ratio of Eu was 0.6 mol % or higher, crystal phaseshaving a composition of Eu_(0.92)(Al_(1.76)Si_(2.24)O₈) formed inaddition to the mullite. Therefore, the upper limit of the content ratioof Eu where Eu can be incorporated in mullite as a solid solution isconsidered to be 0.4 mol % (percentage relative to the total amount(mol) of Al and Si).

INDUSTRIAL APPLICABILITY

Consequently, luminescent apparatuses with any of these examples can beused in, for example, plasma screens, white light-emitting diode andlighting fixtures using the same, and backlight for liquid crystaldisplays.

1. An oxide phosphor comprising: an oxide having a compositionrepresented by formula (1); and an activating element M, wherein M isselected from the group consisting of Ce, Sm, Eu, Yb, Dy and Ag, andwherein the additive amount of M is 0.06 mol % relative to the totalamount(mol) of Al and Si: formula (1) (Al₂O₃)_(x).(SiO₂)_(1-x) wherein xin formula (1) is in the range of:0.5≦x ≦0.7.
 2. The oxide phosphoraccording to claim 1, wherein the oxide, having a compositionrepresented by formula (1), is mullite.
 3. The oxide phosphor accordingto claim 2, wherein the mullite is 3Al₂O₃.2SiO₂, 3Al₂O₃.1.75SiO₂, or3Al₂O₃.1.5SiO₂.
 4. A luminescent apparatus comprising: a phosphor layercomprising the oxide phosphor according to claim 1; and a luminousdevice that generates light exciting the oxide phosphor.